1. Field of the Invention
This invention relates to an ionization smoke detector which detects a fire by detecting a change of an ionization current caused by a change of a smoke density by using a radiation source.
2. Prior Art
When an ionization smoke detector of this type which is installed on a ceiling, it is subject to air stream from a blowout of an air conditioner, it might possibly cause a malfunction because the air stream entering the detector reduces an ionization current within a chamber as by smoke entry. For this reason, a conventional ionization smoke detector generally has a specific smoke intake structure for preventing direct flow of air into the chamber.
As a specific example of the smoke intake structure for preventing possible malfunction, there may be mentioned a structure as disclosed in Japanese Utility Model Application Kokai No. 56-9053. In an ionization smoke detector disclosed in this reference, double partition walls with air inlets formed on respective side wall are provided and the smoke inlets on the outer partition wall and the smoke inlets on the inner partition wall are staggered from each other. Therefore, when the detector is subject to external air stream, it can allow the air stream partially to pass between the outer and inner partition walls, thereby reducing direct air flow into the chamber for preventingt possible malfunction.
With such a structure that the double smoke inlet structure is formed and the smoke inlets are arranged so as to be staggered from each other in the structure, however, direct flow into the chamber would be increased to possibly cause a malfunction when the air stream is strong.
To reduce the influence by the air flow into the chamber when the air stream is hard, an opening area of the smoke inlet formed on the partition wall may be decreased. However, if the opening area of the smoke inlet is decreased, air flow passing between the partition walls is increased to render the interior pressure of the chamber negative. This may possibly be another cause for malfunction. Besides, this causes undesirable insufficiency in detection sensitivity which is lowered due to lack in amount of smoke entering the chamber when a fire occurs.